The present invention relates generally to printers which print a swath of data on a print medium at a time, and pertains more particularly to printmodes for improving the throughput of inkjet printers.
Inkjet printers, and thermal inkjet printers in particular, have come into widespread use in businesses and homes because of their low cost, high print quality, and color printing capability. These devices are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988). The construction and operation of inkjet printers is relatively straightforward, with the basics of the technology further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994)], all of which are incorporated herein by reference. In particular, drops of one or more colored inks are emitted onto a print medium such as paper or transparency film during a printing operation, in response to commands electronically transmitted to one or more printheads in the printer. Inkjet printers may use a number of different ink colors. Each printhead typically emits ink of a different color onto the media. In one commonly used arrangement, the inks are the primary subtractive colors magenta, cyan, and yellow. Alternatively the printer can use more than three color inks, some of which may be lighter and darker versions of a given color shade. Many printers also include a black ink for printing text, and which may also be used during color printing to form the darker shades of colors. The different color inks combine on the print media to form the text and images which are perceived by the human eye. Drops of the color inks can be combined in the same pixels to form a range of perceived colors to the human eye. For example, superimposing drops of magenta and cyan inks in the same location produces a blue color.
One or more printheads for different color inks may be contained in a print cartridge, which may either contain the supply of ink for each printhead or be connected to an ink supply located off-cartridge. An inkjet printer frequently can accommodate two to four print cartridges. The cartridges typically are mounted side-by-side in a carriage which scans the cartridges back and forth within the printer in a forward and a rearward direction with respect to the medium during printing such that the cartridges move sequentially adjacent to given printing locations, called pixels, which are arranged in a row and column format on the medium. Each printhead typically has an arrangement of nozzles through which the ink drops are controllably ejected onto the print medium, and thus a certain width strip of the medium, corresponding to the layout of the nozzle arrangement, can be printed during each scan to form a printed swath. In order to form high quality text and images on the medium, multiple passes of the printhead arrangement back and forth are frequently required to fully print all the pixels of an individual swath. A print medium advance mechanism moves the media relative to the printhead arrangement in a direction generally perpendicular to the movement of the carriage so that, by combining the scans of the print cartridges back and forth across the medium, the emission of ink drops during each scan, and the advance of the medium relative to the printhead arrangement, ink can be deposited on the entire printable area of the medium. The particular combination of scans, ink drop emission during each scan, and the amount and timing of the medium advance used to print on the medium is generally referred to as a xe2x80x9cprintmodexe2x80x9d.
One factor that is very important to purchasers of inkjet printers is the speed at which a page of information can be printed, which in turn relates to the throughput, or the number of pages that can be printed in a given amount of time. Speed and throughput depend upon a number of factors. One of the most significant ones is the number of times that the printhead arrangement must scan an individual swath in order to print all the pixels in the swathxe2x80x94the more scans required, the longer the printing time. The number of scans required depends on the type of information contained in the swath. For example, high quality monochrome (typically black) textual output can typically be produced with a printmode having fewer passes than are required to produce correspondingly high quality color image or color photographic output.
Some printers allow printing a page of information using only a single printmode. Such printers examine the type of information to be printed on the page in order to determine the printmode to be used. If the page contains only textual information of a single color, a monochrome printmode with fewer passes can be used, but if the page contains any color image information a color printmode with more passes must be used and the page will take a longer time to print.
Some other printers have the ability to select the printmode to be used for each individual swath. These printers examine the type of information to be printed on the page on a swath-by-swath basis. For example, if only certain sections of the information contain color images, with the rest of the information being monochrome text, then swaths containing only textual information can be printed using a monochrome printmode with fewer passes, and a color printmode having more passes will be used only for those swaths which contain color image information. Such a single-printmode-per-swath printing scheme improves throughput relative to a printmode-per-page scheme. However, for many printed pages color images make up only a portion of each swath, with monochrome text making up the remainder of the swath. In such situations, the printer throughput is significantly lower than could be achieved if only the image portion of the swath is printed with the greater number of passes of the color printmode, while the text portion of the swath is printed with the fewer number of passes of the monochrome printmode. In addition to reduced throughput, printing the text portions of swaths containing no color images with the monochrome printmode, while printing the text portions of adjacent swaths which do contain color images with the color printmode, can cause visible variations between the adjacent text portions that some users find to be of objectionable print quality. Accordingly, it would be highly desirable to have a new and improved printer and method for printing swaths that prints mixed monochrome and color pages faster and with higher quality.
In a preferred embodiment, the present invention provides a novel method for printing color and monochrome regions of a single swath of halftoned data with different printmodes so as to increase the printing speed of high-quality printed output. According to the novel printing method, the data swath is processed to identify the color and monochrome regions, which typically alternate in the swath. A printhead arrangement is moved relative to a medium in a forward or a backward scan direction over color printing areas of the medium a greater number of times while printing color regions, and a fewer number of times over monochrome printing areas while printing monochrome regions (the number of times is also referred to as a xe2x80x9cscanxe2x80x9d or a xe2x80x9cpassxe2x80x9d). This advantageously reduces the printing time compared to the time would be required if the printhead arrangement was moved over all printing areas the greater number of times. In addition to movement in the scan direction, the printhead arrangement is also periodically advanced relative to the medium in a medium advance direction which is substantially orthogonal to the scan direction. Preferentially the advancing occurs after printing the entire swath of data, but in alternative embodiments the advancing occurs at certain times when the printhead arrangement is reversing direction between the forward and backward scan directions. The preferred method also verifies that a ratio of a size of the monochrome regions to the color regions exceeds a threshold value, in order to assure that any increase in printing time resulting from the more frequent changes in the scan direction are more than offset by the decrease in printing time that results from the use of the lesser number of passes used to print the monochrome regions. If the ratio does not exceed the threshold value, then instead of printing the color and monochrome regions differently, the method treats the swath as if it consists of a single color region, printing it using the greater number of passes. The method has a relationship between the specific numbers of passes used to print the monochrome and color areas. If M passes are used to print monochrome regions of a swath having at least one color region located at a middle portion of the swath between two monochrome regions, then M+2N passes can be used to print the color regions, where N is an integer greater than zero. If M passes are used to print a monochrome region of a swath having only a single color region located at an end portion of the swath, then M+N passes can be used to print the color region if M is odd, and M+2N passes can be used to print the color region if M is even, where N is an integer greater than zero. Approximately 1/Mth of each of each monochrome region is printed during each of the M passes over that region, and approximately 1/Cth of each of each color region is printed during each of the C passes over that region. The method also verifies that the number and locations in the swath of the monochrome and color region or regions are such that using different numbers of M and C passes for the different types of regions provides a faster printing time than using C passes for both types of regions.
If multiple printmodes per swath are utilized for printing a particular swath, a color printmode providing the C passes is used to print the color regions, while a monochrome printmode providing the M passes is used to print the monochrome regions. The appropriate printmode is activated when the printhead arrangement moves across a boundary into a different region. Examining the process of printing the swath in more detail, the region corresponding to the current location in the scan direction of the printhead is selected from the ordered set of regions comprising the swath. A current printmode corresponding to the printmode for the region is activated, and the printhead arrangement is scanned in a current scan direction, emitting drops of ink from the printhead arrangement during scanning, as controlled by the current printmode. When the boundary of the current region is reached, the method determines the next action to take based on the current printmode. This next action will either be to retain the current printmode and reverse the scan direction, or activating the printmode corresponding to the next region in the ordered set and continuing to scan in the same direction.
The monochrome and color printmodes are incorporated in a bidirectional swath printer which is an alternate embodiment of the present invention. The printer includes a frame, a carriage attached to the frame for relative motion with respect to the print medium in oscillating scans along a scan axis, a printhead arrangement mounted to the carriage for controllably depositing drops of different color inks on the print medium during motion of the printhead arrangement, and a print controller operatively connected to the carriage and the printhead arrangement for moving the carriage and depositing the drops. The print controller further includes a data buffer for receiving the data swaths, the monochrome and color printmodes, and an ink deposition controller which activates the monochrome printmode when printing monochrome regions and the color printmode when printing the color regions. Some embodiments of the printer also include a data sorter for receiving the swaths and detecting the monochrome and color regions, a color data plane for receiving the color regions, and a monochrome data plane for receiving the monochrome regions. All pixels in a monochrome region have RGB color attributes of 0,0,0, while at least some pixels in a color region have RGB color attributes of other than 0,0,0. The printer may also include an analyzer coupled to the data buffer and the ink deposition controller for receiving the swaths, analyzing the swaths to determine the state of the multiple-printmode-per-swath control flag that governs whether both color and monochrome printmodes will be used to print the respective regions in the swath, or whether all regions in the swath will be printed using only the color printmode, and communicating the control flag to the ink deposition controller. The printhead arrangement contains at least one print cartridge with at least one ink ejection element array; the axis of the array is orthogonal to the relative motion of the carriage. The printer typically also includes a media advance arrangement attached to the frame and coupled to the print controller for advancing the medium relative to the carriage along an advance axis which is orthogonal to the scan axis.